Not applicable
The invention relates to a motor controller and more particularly to a motor controller for driving a fluid impeller and still more particularly to a motor controller for driving a fluid impeller to provide a specific fluid flow rate.
It is known to employ electric motors to drive fluid impellers such as fan blades or blower cages in air moving apparatus. Such apparatus are typically used in heating, ventilation and air conditioning applications.
It is further known that heating, ventilation and air conditioning systems require a constant fluid flow in order to operate efficiently. Fluid resistance in the ducting of such systems typically varies with time as a result of variations in fluid paths and duct openings. For example, every adjustment of a ventilation opening causes a fluid resistance change in the ducting.
It is known that blower torque must be adjusted to compensate for variable fluid resistance if constant fluid flow is to be maintained.
Various methods and apparatus are known to adjust blower torque in response to variations in fluid resistance or load. Typically, fluid flow may be measured directly by fluid flow transducers which are immersed in the fluid flow path. An electrical signal is typically fed back from the transducers to a microprocessor system or an electric circuit which is designed to adjust the speed of a blower motor to approach a predetermined constant value. Such systems are often too expensive or comprise components that are too large for use in practical heating, ventilation and air conditioning applications.
It is known that the magnitude of phase current in a blower motor drive circuit is related to the magnitude of fluid flow which is impelled by the blower. It is further known to provide a constant flow by comparing a measured phase current of a blower motor drive circuit with an empirically determined ideal reference phase current to determine an error phase current signal. The empirically determined reference phase current value is typically stored in a look-up table in the memory of a microprocessor system. It is further known to manipulate an error phase current signal so that it is suitable for an input as an index to a pulse width modulator in a motor control circuit wherein the motor control circuit is caused to change the motor speed to reduce the error phase current signal. The error phase current signal is reduced as the measured motor current approaches the ideal constant flow reference phase current.
Such methods may provide imprecise control because motor phase currents are known to fluctuate and are typically noisy. Furthermore, such methods require added cost because they typically require motor speed measuring components.
It is desirable to provide a constant fluid flow motor controller of reduced complexity by means that do not require direct measurement of a fluid flow rate, that do not rely on motor phase current measurements and do not require direct motor speed measurements or speed measuring components.
Accordingly, the invention provides a specific fluid flow motor control by operating on critical motor parameters internal to a variable frequency drive. The invention relates to an iterative method of controlling an induction motor to cause a blower to provide a constant selected fluid flow. In at least one embodiment the controller of the invention initially causes a motor to ramp up to an initial operating point. The controller allows the motor/blower system to stabilize at the initial operating point in response to ambient pressure. The controller of the invention then determines a target DC bus current, Idc, that corresponds to a constant selected flow rate at the operating frequency and by employing relation 1.
Idc=fi(Fs, Km, Ai, Bi, Ci, Di . . . )xe2x80x83xe2x80x83Relation 1:
Relation 1 states that the target DC bus current magnitude which will cause a motor blower to produce a constant selected fluid flow rate is a function of stator voltage operating frequency, Fs, a set of one to any number of system specific constants, Ai, Bi, Ci, Di . . . , and a selected fluid flow rate which is represented by parameter Km. In one embodiment of the present invention, the controller of the invention employs a microprocessor system having an electronic memory wherein a table of values of Idc are indexed by required fluid flow rate values and operating frequency. This table is precalculated using the motor/blower system specific constants Ai, Bi, Ci and Di . . . which are empirically determined for the specific motor/blower system. This alternative embodiment comprises a microprocessor system having a memory wherein only the parameters Ai, Bi, Ci, Di . . . are stored. The alternative embodiment further comprises an electronic means such as a digital signal processor which computes values of Idc based on the stored parameters and an empirically determined function (relation 1).
After determining the target DC bus current, Idc, the controller of the invention measures the actual DC bus current, Ia, and determines an error current value which is equal to the difference between the target DC bus current, Idc, and the measured DC bus current, Ia.
The controller of the invention then employs a PI (Proportional Plus Integrating) controller to estimate a new operating stator frequency which will cause the DC bus current to approach the required magnitude. The PI controller is an essential element of the invention.
The controller of the invention then employs relation 2 to determine an operating stator voltage root mean square value, Vs, as a function of the new operating frequency Fs, and a set of one to any number of empirically determined system specific constants, Av, Bv, Cv, Dv . . . .
Vs=fv(Fs, Av, Bv, Cv, Dv . . . )xe2x80x83xe2x80x83Relation 2:
Relation 2 states that the appropriate stator voltage, Vs, is a function of the operating frequency and any number of system specific constant values, Av, Bv, Cv, Dv . . . . The controller of the invention employs a microprocessor system having an electronic memory wherein a table of values of Vs are indexed by the operating frequency only. This table is precalculated using the motor/blower system specific constants Av, Bv, Cv, Dv . . . , which are empirically determined for the specific motor/blower system. An alternative embodiment comprises a microprocessor system having a memory wherein only the parameters Av, Bv, Cv, Dv . . . are stored. This alternative embodiment further comprises an electronic means such as a digital signal processor which computes values of Vs based on the stored parameters and an empirically determined function (relation 2).
The new operating stator frequency and stator voltage root mean square are provided to the variable frequency drive. The controller of the invention then allows a stabilization time delay to pass before beginning another iteration of the method.
After each stabilization period, the controller of the invention begins another iteration by determining a target DC bus current using relation 1. Then the controller of the invention completes the iteration by measuring the actual DC bus current, calculating the current error, estimating a new stator operating frequency, determining the appropriate voltage at the new stator frequency and loading the new stator operating voltage and frequency values into the pulse width modulator.